As described in Japanese Patent Application (OPI) No. 68246/79, a dispersion type carrier comprising a magnetic substance dispersed in a resin is shown to be excellent in reproductivity of solid images and line images when used as a carrier for a two-component type developer to be used in development of an electrostatic latent image.
It is also known that a coating layer can be provided on the above dispersion type carrier and an electron charge-controlling agent is added to the coating layer to control charging (see, for example, Japanese Patent Application (OPI) No. 59457/83).
The term "OPI" as used herein means an "unexamined published patent application".
However, such dispersion type carrier with a coating layer provided thereon has a number of problems. First since the shape of a core- material is not definite, a uniform coating layer is difficult to form. Also, a solvent-soluble polymer cannot be used as the binder for the core. Further, the surface of the carrier is worn by use over a long period of time and the coating layer is peeled apart, which increases the changes in the amount of electric charge on the carrier surface.
Another problem is that a toner attaches to the carrier surface (toner impaction), which causes a reduction in the amount of electric charge on the carrier after long periods of use. In order to prevent this toner impaction, it has been proposed to use polyolefin having a low surface energy as a coating layer on the carrier surface, as described in U.S. patent application Ser. No. 148,345 filed Jan. 25, 1988. However, when polyolefin with an electric charge controlling agent added thereto is used as a coating layer, the electric charge controlling agent is not uniformly dispersed in the polyolefin. Consequently, when the carrier is used for a long time, the electric charge controlling agent tends to drop from the carrier. In order to overcome the latter problem, it has been proposed to prevent the dropping from the carrier surface by using an electric charge controlling polymer.
However, polyolefin has poor compatibility with other electric charge controlling polymers. Therefore, when it is incorporated in the coating layer in the form of a mixture with electric charge controlling polymers, phase separation occurs in the coating layer and there is formed a coating layer in which the polyolefin and the electric charge controlling agent are not uniformly dispersed. Therefore, there is a problem in that a large variation occurs in the amount of electric charge on the carrier surface.